Environmental mutagens induce mutations in prokaryotes and presumably also in eukaryotes in a non-random fashion with respect to the DNA sequence. An understanding of the non-random character and the underlying mechanism of induced mutagenesis, will eventually lead to better risks estimates for mutagens present in the environment. To investigate the mutational specificity in mammalian cells at the molecular level, the adeninephosphoribosyltransferase (aprt) gene in a cell-line hemizygous for aprt was chosen as a model. This gene was elected in view of its small size (less than 2.5 kb, including all regulatory sequences and four introns) and the well-established selective growth conditions for aprt-deficient cells. Special lambda vectors have been developed to allow the efficient and repetitive isolation of mutant alleles for DNA sequence analysis. A large number of mutant alleles have now been processed through these cloning protocols: 58 resulting from spontaneous mutations, 40 induced by gamma ray and 44 induced by UV. For a considerable fraction of the cloned genes, the mutational changes have been elucidated. Spontaneous and UV-induced mutations are, indeed, spread across the gene in a non-random way. Three specific transition hot-spots are observed among the spontaneous mutants and a transversion hot-spot is suggested by data collected for UV-induced mutants.